Operation of lean burn engines such as diesel engines provide the user with excellent fuel economy and have very low emissions of gas phase hydrocarbons and carbon monoxide due to their operation at high air/fuel ratios under fuel lean conditions. Diesel engines also offer significant advantages over gasoline engines in terms of their fuel economy, durability, and their ability to generate high torque at low speed. However, there are certain materials contained in diesel engine exhaust gas which are known to cause pollution and therefore may have severe influence on the environment. Apart from gaseous emissions such as carbon monoxide (“CO”), unburned hydrocarbon (“HC”) and nitrogen oxides (“NOx”), diesel engine exhaust also contains condensed phase materials, i.e. liquids and solids, which constitute the so-called particulate matter (“PM”). The total particulate matter emissions comprised in diesel exhaust comprises, apart from the soluble organic fraction (“SOF”) and the so-called the sulfate fraction, the solid and dry carbonaceous fraction which is also known as “soot” fraction. This soot contributes to the visible soot emissions commonly associated with diesel exhaust. The soluble organic fraction can exist in diesel exhaust either as vapour or as an aerosol, i.e. fine droplets of liquid condensate, depending on the temperature of the diesel exhaust. Generally, it is present as condensed liquid at the standard particulate collection temperature of 52° C. in diluted exhaust, as prescribed by a standard measurement test such as the U.S. Heavy Duty Transient Federal Test Procedure. These liquids are believed to arise from two sources: one the one hand, lubricated oil swept from cylinder walls of the engine each time the pistons go up and down, and on the hand, unburned or only partially burned diesel fuel. The sulfate fraction is believed to be formed from small quantities of sulfur components present in the diesel fuel.
Catalyzed filters are typically provided in diesel engine exhaust systems to achieve high particulate matter reduction, in particular soot reduction, and to convert certain or all of the exhaust components to innocuous components. Known filter structures that remove particulate matter from diesel exhaust include honeycomb wall flow filters, wound or packed fiber filters, open cell foams, sintered metal filters, etc. However, ceramic wall flow filters, described below, receive the most attention. Typical ceramic wall flow filter substrates are composed of refractory materials such as cordierite or silicon-carbide. Wall flow substrates are particularly useful to filter particulate matter from diesel engine exhaust gases. A common construction is a multi-passage honeycomb structure having the ends of alternate passages on the inlet and outlet sides of the honeycomb structure plugged. This construction results in a checkerboard-type pattern on either end. Passages plugged on the inlet axial end are open on the outlet axial end. This permits the exhaust gas with the entrained particulate matter to enter the open inlet passages, flow through the porous internal walls and exit through the channels having open outlet axial ends. The particulate matter is thereby filtered on the internal walls of the substrate. The gas pressure forces the exhaust gas through the porous structural walls into the channels closed at the upstream axial end and open at the downstream axial end. The accumulating particles will increase the back pressure from the filter on the engine. Thus, the accumulating particles have to be continuously or periodically burned out of the filter to maintain an acceptable back pressure. Catalyst compositions deposited along the internal walls of the wall flow substrate assist in the regeneration of the filter substrates by promoting the combustion of the accumulated particulate matter. The combustion of the accumulated particulate matter restores acceptable back pressures within the exhaust system. These processes may be either passive or active regeneration processes. Both processes utilize an oxidant such as O2 or NO2 to combust the particulate matter. Passive regeneration processes combust the particulate matter at temperatures within the normal operating range of the diesel exhaust system. Preferably, the oxidant used in the regeneration process is NO2 since the soot fraction combusts at much lower temperatures than those needed when O2 serves as the oxidant. While O2 is readily available from the atmosphere, NO2 can be actively generated though the use of upstream oxidation catalysts which oxidize NO in the exhaust stream.
In spite of the presence of the catalyst compositions and provisions for using NO2 as the oxidant, active regeneration processes are generally needed to clear out the accumulated particulate matter, and restore acceptable back pressures within the filter. The soot fraction of the particulate matter generally requires temperatures in excess of 500° C. to burn under oxygen rich, i.e. lean conditions, which are higher temperatures than those typically present in diesel exhaust. Active regeneration processes are normally initiated by altering the engine management to raise temperatures in front of the filter up to 570-630° C. Depending on driving mode, high exotherms can occur inside the filter when the cooling during regeneration is not sufficient, such as at low speed/low load or in idle driving mode. Such exotherms may exceed 800° C. or more within the filter. In coated wall flow filters, exposure to such high temperatures during regeneration events shortens the useful lifetime of the catalyst compositions coated along the length of the substrate. Moreover, different segments along the axial length of the substrate are disproportionately affected by the regeneration process. Deposition of the particulate matter is not homogeneous along the length of the wall flow filter, with higher proportions of the particulate matter accumulating in the downstream segment of the filter. Consequently, the temperatures are not homogeneously distributed over the length of the substrate but show a maximum temperature in the downstream segment during active regeneration events. Thus, the durability of the catalyst composition along the downstream segment limits the useful lifetime of the entire catalyst-coated wall flow substrate.
High material costs associated with certain oxidation catalysts such as, for example, platinum group metal-containing compositions augment the need to slow or prevent the degradation of catalyst coatings due to active regeneration events. Catalyst coatings disposed on wall flow filters often contain platinum group metal components as active catalyst components to ensure acceptable conversions of the gaseous emissions such as HC and/or CO of the diesel exhaust to innocuous components (e.g., CO2, H2O). The loadings of such components are generally adjusted so that the catalyst substrate meets emissions regulations even after catalyst aging.
Certain conventional coating designs for wall flow substrates have a homogeneous distribution of coating along the entire axial length of the internal walls. In such designs the oxidation catalyst concentration is typically adjusted to meet the emissions requirements under the most stringent conditions. Most often such conditions refer to the catalyst's performance after the catalyst has aged. The cost associated with the required platinum group metal concentration is often higher than is desired.
Other conventional coating designs for wall flow substrates employ concentration gradients of the platinum group metal components along the axial length of the substrate. In these designs certain catalyst zones, e.g., an upstream zone, have a higher concentration of platinum group metals than do adjacent axial zones such as, e.g., a down-stream zone. Typically, the internal walls of the axial zone where the higher concentration of platinum group metal components are disposed, will have a lower permeability than an adjacent zone having a lower concentration of platinum group metals due to a higher washcoat loading. An exhaust stream passing along the length of the inlet passage will travel through the internal wall in the segments that have the highest permeability. Thus, the gas stream will tend to flow through the internal wall segments that have lower concentration of oxidation catalyst. This differential flow pattern can result in inadequate pollutant conversion. For instance, certain gaseous pollutants, e.g., unburned hydrocarbons, require contact with higher concentrations of platinum group metal components than do particulate components to achieve sufficient levels of combustion. This requirement is exacerbated during operating conditions where the exhaust temperatures are cooler, e.g., at startup.
EP 1 870 573 A1 discloses a diesel particulate filter which comprises a plurality of cells which are partitioned by porous cell walls and are closed in a staggered manner by plugs at an upstream end of the filter and at an opposite downstream end thereof wherein an first oxidation catalyst coating layer is formed on the entire surface of the cell walls of the cells that are open at the upstream end of the filter, and a second oxidation catalyst coating layer is formed on the surfaces of the cell walls of the cells which are open at the downstream end of the filter, in a downstream part of the filter. Thus, this document discloses filters having a region of the cell walls dividing the cells which are open at the upstream end and the cells which are open at the downstream end wherein the catalyst coating layers of the respective cells overlap, due to the fact that the first oxidation catalyst coating layer is formed on the entire surface of the respective cell walls.
WO 01/12320 A1 discloses a wall-flow filter for an exhaust system of a combustion engine, which filter comprises a plurality of channels in honeycomb arrangement, wherein at least some of the channels are plugged at an upstream end and at least some of the channels not plugged at the upstream end are plugged at a downstream end; an oxidation catalyst on a substantially gas impermeable zone at an upstream end of the channels plugged at the downstream end; and a gas permeable filter zone downstream of the oxidation catalyst for trapping soot, characterized in that in an exhaust system the oxidation catalyst is capable of generating sufficient NO2 from NO to combust the trapped soot continuously at a temperature less than 400° C. According to this document, the coatings on the opposite sides of a given cell wall are applied in such a way that there is a region of the cell wall which is free of coating on both sides in order to allow for a gas permeable zone.
EP 1 486 248 A1 discloses an integrated multi-functional catalyst system which comprises a diesel particulate filter having an inlet side for receiving flow and an opposite outlet side; a substrate in the diesel particulate filter having an interior wall surface and an exterior wall surface; a first washcoat layer applied to the interior wall surface and adjacent the inlet side; and a second washcoat layer applied to the exterior wall surface and adjacent the outlet side, wherein flow distribution through the substrate is dispersed for minimizing back pressure. According to a preferred embodiment, the first washcoat layer occupies a first length of the substrate, the second washcoat layer occupies a second length of the substrate, wherein the sum of the first length and the second length is approximately equal to a total length of the substrate. According to a still further preferred embodiment, EP 1 486 248 A1 discloses a diesel particulate filter having an inlet side for receiving flow and an opposite outlet side; a plurality of honeycomb cells within the diesel particulate filter, wherein alternating exit channels are blocked at the inlet side and alternating inlet channels are blocked at the opposite outlet side; a substrate for each of the inlet channels, each substrate having an interior wall surface and an exterior wall surface; a first washcoat layer applied to the interior wall surface and adjacent the inlet side; and a second washcoat layer, applied to the exterior wall surface and adjacent the outlet side, wherein flow distribution through the substrate is dispersed for minimizing back pressure. It is stated that this second washcoat layer contains a different function than the first washcoat layer.
WO 2006/031600 A1 discloses a zoned catalyzed soot filter having a wall flow substrate with an inlet end, an outlet end, a substrate axial length extending between the inlet end and the outlet end, and a plurality of passages defined by internal walls of the wall flow substrate. The plurality of passages has inlet passages with an open inlet end and a closed outlet end, and outlet passages with a closed inlet end and an open outlet end. The internal walls of the inlet passages have a first inlet coating that extends from the inlet end to a first inlet coating end, thereby defining a first inlet coating length. The first inlet coating length is less than the substrate axial length. The internal walls of the outlet passages have an outlet coating that extends from the outlet end to an outlet coating end, thereby defining an outlet coating length. The outlet coating length is less than the substrate axial length. The sum of the first inlet coating and outlet coating lengths is substantially equal to the substrate axial length. The first inlet coating length defines an upstream zone and the outlet coating length defines a downstream zone. The first inlet coating contains at least one first inlet platinum group metal component. At least 50% of the platinum group metal components are present in the upstream zone. According to the teaching of this document, the washcoat loading ratio, defined as the washcoat loading of the first inlet zone relative to the washcoat loading of the outlet coating, is in the range from 0.5 to 1.5. Thus, this document does not differentiate between embodiments wherein the washcoat loading ratio is greater than or smaller than 1. Further, a specific lower limit for the washcoat loading ratio is defined, namely a limit of 0.5.
Generally, when an active regeneration of a catalyzed soot filter used in a diesel exhaust system is stopped during its run, e.g. when the engine goes into idle run, very high temperatures occur in the rear end of the catalyzed soot filter by uncontrolled soot burning. It is believed that the temperature maximum in that rear part of the filter decreases with the soot loading in the rear part of the filter. The maximum soot loading on the filter, often referred to as soot mass limit (“SML”) is determined by this maximum temperature. It was an object of the present invention to provide a catalyzed soot filter which allows for an increased soot mass loading (“SML”) and thus for a decreased maximum temperature.
Therefore, the present invention is directed to a catalyzed soot filter which has a coating design which allows for a low maximum temperature during drop to idle regeneration and a high soot mass limit.